Multi-component composite membrane and method for preparing the same

ABSTRACT

The present invention relates to a multi-component composite separate membrane and a method for preparing the same, and to a multi-component composite membrane comprising a support layer and active layers, wherein the support layer is located between the active layers.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on application No. 2000-34948 filed in theKorean Industrial Property Office on Jun. 23, 2000, the content of whichis incorporated hereinto by reference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a multi-component composite membraneand a method for preparing the same, and more particularly to amulti-component composite membrane comprising a support layer and anactive layer having a structure as dense as the conventional activelayers, which is capable of having pores formed thereon according toprocess conditions and with improved permeability due to the pores of acontrollable size, the composite membrane having characteristics of anactive layer, and with an interfacial adhesion strength between thesupport layer and the active layer strengthened by ion beamsirradiation, and a method of preparing the same.

(b) Description of the Related Art

Many types of membranes are currently in use, such as microfiltrationmembranes, ultrafiltration membranes, gas separation membranes,pervaporation membranes, and reverse osmosis membranes.

The present invention relates to a microfiltration membrane, and inparticular to a separator comprising polyolefins such as polyethyleneand polypropylene, for a rechargeable lithium ion battery.

As one of the polyolefins, when high crystalline polypropylene (HCPP) isused for a separator of the present invention, it is expected that thepermeability of the separator will increase. The crystallinity of commonpolypropylenes is less than 50%, but the crystallinity of HCPP isgreater than 50% and it is highly isotactic, so that density, meltingpoint, heat of fusion, and crystallization temperature are high, andcharacteristics such as rigidity, heat-resistance, impact strength,scratch-resistance, and dimensional stability are excellent.

A composite membrane is generally prepared by interfacialpolymerization, modification of membranes, and dip coating. Dip coatingis widely used in order to prepare the composite membrane, by using amicroporous membrane such as a microfiltration membrane or anultrafiltration membrane as a support layer, coating the microporousmembrane with a solution of a material used as an active layer, anddrying the coated membrane. The composite membrane prepared by dipcoating has a support layer comprising regularly-sized pores, and anactive layer having a dense structure with few pores. The compositemembrane is limited in application, since the active layer has few poresof a size similar to those of the microfiltration or ultrafiltrationmembranes, and it is easily delaminated due to a weak interfacialadhesion strength between the support layer and the active layer.

The composite membrane may be prepared by coating a polymer on themicroporous membrane as disclosed in U.S. Pat. Nos. 3,249,109,4,388,189, and 5,102,552. In addition, a hydrophilic monomer, such as anacrylic acid, and polymers such as polyethylene oxide are grafted withcorona treatment so that the membrane has a modified surface, and inparticular so that it has hydrophilicity as disclosed in U.S. Pat. Nos.4,346,142, 5,085,775, and 5,294,346. However, though the membrane has amodified surface and hydrophilicity, the method of graft polymerizationis not applied, since the process is complicated and permeability of themembrane is not satisfactory.

A separator having regularly-sized pores for a common battery is coatedwith a polymer electrolyte solution, and it is used as a separator for arechargeable lithium ion battery as disclosed in U.S. Pat. No. 5,716,421and European Patent No. 0933824A2. However, when the separator isprepared by the aforementioned method, the membrane has a densestructure, that is, no pores are formed on the-surface of the membrane,and permeability (e.g. air permeability) deteriorates, and theinterfacial adhesion strength between the support layer and the activelayer is inadequate.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a multi-componentcomposite membrane comprising a support layer and an active layer havinga structure as dense as the conventional active layers, which is capableof having pores formed thereon according to process conditions and withimproved permeability due to the pores of a controllable size, thecomposite membrane having characteristics of an active layer, and withan interfacial adhesion strength between the support layer and theactive layer strengthened by ion beams irradiation, and a method ofpreparing the same.

In order to accomplish the object, the present invention provides amulti-component composite membrane comprising a support layer and twoactive layers.

Furthermore, the present invention provides a preparation method of amulti-component composite membrane comprising the steps of:

-   -   a) preparing a precursor film by injection of a polymer, which        is used for a support layer, into an extruder;    -   b) annealing the precursor film at a temperature less than a        melting point of the polymer;    -   c) irradiating ion beams on either or both surfaces of the        annealed precursor film with the help of an reactive gas;    -   d) coating both surfaces of the irradiated precursor film with a        polymer solution, which is used for an active layer;    -   e) drying the coated precursor film;    -   f) low temperature-stretching the dried precursor film at a        temperature less than a room temperature;    -   g) high temperature-stretching the low temperature-stretched        precursor film at a temperature less than a melting point of the        polymer; and    -   h) heat setting the high temperature-stretched precursor film        with a tension at a temperature less than the melting point of        the polymer.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention, and many of the attendantadvantages thereof, will be readily apparent as the same becomes betterunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings, wherein:

FIG. 1 is a scanning electron microscope (SEM) photograph showing asurface of a composite membrane of Example 1 according to the presentinvention; and

FIG. 2 is a SEM photograph showing a surface of a conventional compositemembrane according to Comparative Example 1.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, only the preferred embodiment ofthe invention has been shown and described, simply by way ofillustration of the best mode contemplated by the inventors of carryingout the invention. As will be realized, the invention is capable ofmodification in various obvious respects, all without departing from theinvention. Accordingly, the drawings and description are to be regardedas illustrative in nature, and not restrictive.

The present invention is described in detail as follows.

The present invention provides a composite membrane and a preparationmethod for the same that involves coating a common film having no poreswith an active material, instead of as in the conventional method ofcoating a microporous film with an active material.

The composite membrane of the present invention is prepared by joint-useof a conventional dry process in which the pores are formed bystretching, and a phase inversion that is used with a solution. Inaddition, when an ion beam irradiation step is added in the preparationsteps to prepare the membrane of the present invention, an interfacebond between a support layer and an active layer is improved.

The preparation method using a conventional dry process is a method inwhich pores are formed by rupturing a relatively weak amorphous regionthrough cold stretching after orientating a polymer crystalline regionin a certain direction, and the orientation uniformity of thecrystalline region is critical for characteristics of the membrane.

The method using phase inversion is a method in which pores are formedby phase-separation of a polymer and a solvent from the solution under acontrolled temperature, or the use of a non-solvent after preparing apolymer solution.

In order to modify the surface, an ion beam irradiation process is usedin which gases such as gaseous argon, hydrogen, oxygen, nitrogen, andcarbon dioxide are ionized and irradiated to the surface under anatmosphere of reactive gases to be reacted with the ions and the surfacewhen the ionized gases collide with the surface of the membrane.

In the present invention, in order to prepare a material used as asupport layer, a precursor film is prepared in one step of the dryingprocess, it is coated with a polymer solution used for an active layer,it is phase-separated from the polymer solution under suitableconditions, and it is stretched, and thereby the membrane is preparedand pores are formed on the membrane. During the membrane preparation,in order to increase an interfacial adhesion strength between thesupport layer and the active layer, the ion beam irradiation process isperformed before the coating process, so that the membrane surface ismodified. The composite membrane of the present invention comprisesmaterials having pores, which are used for the support layer and theactive layer, respectively. The pore size and distribution of thesupport layer and active layer are different from each other, with thepores of the support layer being formed by a stretching process afterorientating a polymer crystalline region in a certain direction duringthe precursor film preparation. The pores of the active layer, on theother hand, are formed by a stretching process after forming a denselystructured polymer film through phase-inversion. Micro-cracks andmicro-pores of the polymer film can be formed according to thephase-inversion conditions before the film is stretched, so the degreeof pore formation is controllable according to said phase-inversionconditions.

The support layer of the present invention has the same characteristicsas a membrane prepared from the conventional dry process, and the activelayer has pores with various sizes according to the process conditions.In addition, inter-diffusion among the polymer chains of the supportlayer and active layer improves through high temperature-stretching andheat-setting, and the surface bond between the support layer and theactive layer strengthens. when ion beams are irradiated to the layers,the surface bond may further strengthen.

The material used for the support layer of the present invention is notlimited to a certain material, and it generally includes one or morematerials selected from the group consisting of high densitypolyethylene, low density polyethylene, linear low density polyethylene,polypropylene, high crystalline polypropylene, polyethylene-propylenecopolymer, polyethylene-butylene copolymer, polyethylene-hexenecopolymer, polyethylene-octene copolymer,polystyrene-ethylene-butylene-styrene copolymer, polystyrene,polyphenylene oxide, polysulfone, polycarbonate, polyester, polyamide,polyurethane, polyacrylate, polyvinylidene chloride, polyvinylidenefluoride, polysiloxane, polyolefin, ionomer, polymethylpentene, andhydrogenated oligocyclopentadiene (HOCP), and a mixture thereof, andpreferably only material, blended material, or laminated materialselected from the aforementioned group is used.

The polymer of the polymer solution used for the active layer isselected according to the eventual use of the composite membrane, and itpreferably includes at least one material selected from the groupconsisting of polyethylene, polypropylene, polyvinylidene fluoride,polyvinylidene fluoride-hexafluoropropylene copolymer, polyethyleneoxide, polypropylene oxide, polybutylene oxide, polyurethane,polyacrylonitrile, polyacrylate, polyacrylic acid, polyamide,polyacrylamide, polyvinylacetate, polyvinylpyrrolidone,polytetraethylene glycol diacrylate, polysulfone, polyphenylene, oxide,polycarbonate, polyester, polyvinylidene chloride, polysiloxane, and apolyolefin inomer, and a derivative thereof.

The solvent of the polymer solution is selected according to the polymerused, and it preferably includes at least one solvent selected from1-methyl-2-pyrrolidone (NMP), acetone, ethanol, n-propanol, n-butanol,n-hexane, cyclohexanol, acetic acid, ethyl acetate, diethyl ether,dimethyl formamide (DMF), dimethylacetamide (DMAc), dioxane,tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), cyclohexane, benzene,toluene, xylene, and water, and a mixture thereof.

The polymer solution is preferably used under the following conditions.During the coating process of the polymer solution, a common film havingno pores is dip-coated in the polymer solution, with the concentrationof the polymer solution preferably being 0.01 wt % or greater. Inaddition, it is preferable that the drying of the, coated polymer isperformed at a relative humidity ranging from 1 to 100% under anatmosphere of a gas selected from gases comprising nitrogen, oxygen,carbon dioxide, and air at a saturated vapor pressure of less than asaturated vapor pressure of the solvent. The thickness of the activelayer after coating and drying preferably ranges from 0.1 to 20 μm.

The ion beam irradiation is performed under a vacuum ranging from 10⁻¹to 10⁻⁶ torr, with activated, electrons, hydrogen, helium, oxygen,nitrogen, carbon dioxide, air, fluorine, neon, argon, krypton, and N₂O,and a mixture thereof, the aforementioned ion particles having an energyranging from 0.01 to 10⁶ keV. Preferably, the amount of the ion particleranges from 10⁵ to 10²⁰ ions/cm². The reactive gases preferably includehelium, hydrogen, oxygen, nitrogen, ammonia, carbon monoxide, carbondioxide, chlorofluorocarbon, methane, and N₂O, and a mixture thereof,and the flow rate of the reactive gases preferably ranges from 0.5 to 20ml/minute.

The preparation method of the composite membrane according to thepresent invention comprises the following steps.

-   -   a) Precursor film preparation of the support layer: A precursor        film is prepared by extrusion of a polymer used for a support        layer with an extruder equipped with a T-die or tubular die.    -   b) Annealing: The precursor film is annealed in a dry oven at a        temperature lower than a melting point of the polymer so that        the precursor film has an increased crystallinity and ratio of        elastic recovery.    -   c) Irradiation of ion beams with the help of an reactive gas:        After the precursor film is placed in a vacuum chamber and        ionized gas is injected into an ion gun so that the gas has an        energy, the ion particles having an energy are irradiated on        either or both sides of the precursor film, depending on various        currents. A power source is controlled in order that the energy        of the ions ranges from 0.01 to 10⁶ keV. while irradiating the        ion beams, an reactive gas of which flow rate is varied from 0.5        to 20 ml/minute is injected into the vacuum chamber in order to        modify a surface of the precursor film. The modification of the        surface of the precursor film may be performed before or after        annealing, depending on desired physical properties of the        multi-component composite membrane.    -   d) Coating the precursor film with a polymer solution used for        an active layer: After a polymer solution is prepared by        dissolving a polymer used for an active layer in a desired        solvent, the precursor film is coated with the polymer solution.        The precursor film may be used before or after annealing. In        addition, before coating, the irradiation of ion beams with the        help of the reactive gas may be performed, depending on the        physical properties of the multi-component composite membrane.        Concentration and coating conditions may be varied according to        materials used and the eventual use of the composite membrane.    -   e) Formation of a polymer film by phase-inversion: After        coating, the solvent is vaporized under suitable conditions. The        structure of the polymer film of the active layer depends on the        drying conditions.    -   f) Low-temperature stretching: Microcracks are formed by        mono-axially stretching the annealed film with use of a roll or        other different stretching machines at a temperature lower than        room temperature.    -   g) High-temperature stretching: Micropores having desirable        sizes are formed and mechanical properties are provided to a        membrane by the ion-beam-irradiation and mono-axially or        bi-axially stretching the low temperature stretched film with        use of a roll or other machine at a temperature lower than a        melting point of the polymer of the support and active layers.    -   h) Heat-setting: After the high-temperature stretching, the film        is heat-set at a temperature lower than a melting point of the        polymer of the support and active layers under tension, for a        certain time.

The preparation steps of the multi-component composite membrane describethe overall processes for preparing a membrane having optimum physicalproperties, wherein the membrane can be prepared not only by skippingsome of the steps or adding processes depending on physical properties,but also by changing the sequence of each of the steps.

The following Examples and Comparative Examples illustrate the presentinvention in further detail, but the present invention is not limited bythese examples.

The microporous membranes prepared by the following Examples andComparative Examples were evaluated according to the followingcharacteristics:

-   -   a) thickness;    -   b) air permeability: JIS P8117;    -   c) pore size: scanning electron microscope (SEM), transmission        electron microscope (TEM);    -   d) interfacial adhesion strength: JIS Z0237; and    -   e) a wet-out rate of electrolyte (electrolyte used: ethylene        carbonate (EC):dimethyl carbonate (DC)=4:6)

EXAMPLE 1 Composite Membrane Prepared from High CrystallinePolypropylene and Kynar461

A high crystalline polypropylene was used for a support layer and apolyvinylidene fluoride (PVDF) was used for an active layer in order toprepare a precursor film, and the precursor film was stretched through adry process in order to prepare a composite membrane.

(Preparation of a Precursor Film)

High crystalline polypropylene was used for a component of a supportlayer. It has a melting index of 0.50 g/min, a density of 0.91 g/cc, amelting point of 166.5° C. measured with a dynamic scanning calorimeter(DSC), a crystallization temperature of 134.5° C., a crystallinity of57%, isotacticity of 98% measured by C¹³ nuclear magnetic resonance(NMR), and an atactic fraction of about 2% measured after dissolution inxylene, and a precursor film was prepared from the high crystallinepolypropylene with use of a single screw extruder equipped with T-dieand a take-up device. Extrusion temperature and cooling-roll temperaturewas 220° C. and 80° C. respectively, take-up speed was 20 m/min, and adraw down rate (DDR) was 60.

(Annealing)

The prepared precursor film was annealed in a dry oven at 150° C. for 1hour.

(Coating)

After annealing, a solution prepared by dissolving Kynar461 (a productby Elf Atochem North America Co.) having low crystallinity in acetonewas dip-coated on both sides of the prepared precursor film. The coatingwas performed under air while maintaining 60% relative humidity, and theacetone was vaporized at the same condition of 60% relative humidity.Thickness of the coated Kynar461 was about 3 μm.

(Low-Temperature Stretching)

After coating, the coated film was mono-axially low-temperaturestretched to 50% of the stretching ratio based on the initial length ofthe film at room temperature.

(High-Temperature Stretching)

After low-temperature stretching, the low-temperature-stretched film wasmono-axially high-temperature stretched to 100% of the stretching ratiobased on the initial length of the film, at 140° C.

(Heat-Setting)

After the high-temperature stretched film was heat-set at 140° C. undertension for 10 minutes, a composite membrane was prepared by cooling theheat-set film.

Properties of the composite membrane are represented in Table 1.

It is shown in Table 1 that micropores form on an active layer as wellas a support layer according to Example 1, and air permeability of themembrane according to Example 1 improves, compared to no microporesforming on the Kynar461: used for an active layer (See FIG. 1 and FIG.2) when the Kynar461 was coated on a separator as in the conventionalpreparation method. In addition, the interfacial adhesion strengthbetween the support layer and the active layer increased. It is supposedthat the wet-out rate of electrolyte increased due to the change ofmorphology and the increase of adhesion strength.

EXAMPLE 2 A Composite Membrane Prepared from High CrystallinityPolypropylene and Kynar461 with Irradiation of Ion Beams

A composite membrane was prepared by same method of Example 1, exceptthat ion beams were irradiated on a precursor film before coating withthe Kynar461 solution. After the precursor film prepared in the samemanner as in Example 1 was placed in a vacuum chamber while keeping thepressure, ranging from 10⁻⁵ to 10⁻⁶ torr, argon cations were irradiatedto both sides of the precursor film with an ion gun, and simultaneouslyoxygen used as an reactive gas was injected into the chamber in anamount of 4 ml/min in order to treat the precursor film surface. Energyof the ion beams was 0.5 keV, and the irradiation amount of ions was10¹⁶ ions/cm². After the ion-beam irradiation, a composite membrane wasprepared in the same manner as in Example 1.

In Table 1, it is shown that pores were formed on both the support layerand the active layer as in Example 1, and in particular, the interfacialadhesion strength between the support layer and the active layer, andthe wet-out rate of the electrolyte were appreciably improved.

EXAMPLE 3 A Membrane Prepared from High Density Polyethylene/Kynar461

A composite membrane was prepared in the same manner as in Example 1,except that high density polyethylene was used for a support layerinstead of high crystalline polypropylene. The high density polyethylenehad a melt index of 0.3 g/10 min and a density of 0.964 g/cc. Aprecursor film was prepared in the same manner as in Example 1. Theextrusion temperature and cooling-roll temperature of the take-up devicewere respectively 200° C. and 80° C., the take-up speed of the film was30 m/min, and the draw-down ratio of the prepared precursor film was 80.The prepared precursor film was annealed in a dry oven at 125° C. for 1hour. Both sides of the annealed precursor film were coated withKynar461 in the same manner as in Example 1. The coated precursor filmwas mono-axially stretched at room temperature to 50% of the stretchingratio based on the initial length of the film, and then it was,immediately mono-axially high-temperature stretched to 50% of thestretching ratio based on the initial length of the film, at 120° C. Thehigh-temperature stretched film was heat-set at 120° C. under tensionfor 10 minutes, and then a composite membrane was prepared by coolingthe heat-set film. Table 1 shows properties of the prepared compositemembrane.

In Table 1, it is observed that pores formed on both the support layerand the active layer as in Example 1, and the interfacial adhesionstrength and wet-out rate of the electrolyte improved.

Comparative Example 1 A Composite Membrane Prepared from Celgard2400 andKynar461

An active layer was coated on a microporous membrane by the conventionalmethod.

Celgard2400 (a product by Celanese Co.) prepared from only polypropylenewas used for the porous membrane as a support layer, Kynar461 was usedfor an active layer as in Examples 1, 2 and 3, and the Kynar461 solutionwas coated on the Celgard2400 having pores, and thereby a compositemembrane was prepared.

FIGS. 1 and 2 show that the composite membrane of Comparative Example 1has no pores, unlike the composite membrane of the examples according tothe present invention that have pores formed on the active layer.

Table 1 shows properties of the prepared composite membranes. It isshown that the composite membrane prepared from Celgard2400 and Kynar461had an air: permeability too inferior to measure, and the interfacialadhesion strength and wet-out rate of electrolyte were inferior.

TABLE 1 Comparative Example 1 Example 2 Example 3 Example 1 Thickness(μm) 20 20 20 20 Pore Support 0.3 × 0.1 0.3 × 0.1 0.4 × 0.1 0.3 × 0.1size layer (μm) Active 0.8 × 0.3 0.8 × 0.3 0.6 × 0.3 Unable to be Layermeasured Air permeability 560 565 620 Unable to be (sec/ 100 cc)measured Interfacial 180 250 240 85 adhesion strength (g_(f)) Wet-outrate of 10 8 9 45 an electrolyte (sec)

The composite membrane prepared by the conventional method has apermeability that is too inferior to be measured, but the compositemembranes of the present invention have an improved air permeabilityranging from 560 to 620 sec/100 cc, because both the active layer andthe support layer have a dense structure with pores of a controllablesize prepared under suitable preparation conditions. In addition, theactive layer located on the exterior side of the composite membrane hasgood properties. That is, the support layer of the present invention hasthe same properties as the membrane prepared by the conventional dryprocess, and the active layer has pores with various sizes according tothe process condition.

In addition, while the composite membrane prepared by the conventionalmethod has an interfacial adhesion strength of 85 g_(f), the compositemembrane of the present invention has an improved interfacial adhesionstrength ranging from 180 to 250 g_(f). The improved interfacialadhesion strength results from high-temperature stretching andheat-setting, that is, the interfacial adhesion strength increasesbecause the mutual bond between polymer chains of the support and activelayers strengthens. The interfacial adhesion strength improves furtherby irradiation of ion beams.

Furthermore, the wet-out rate improves appreciably, and it is supposedthat the improvement of the wet-out rate is due to changes of morphologyand an increase of the interfacial adhesion strength.

While the present invention has been described in detail with referenceto the preferred embodiments, those skilled in the art will appreciatethat various modifications and substitutions can be made thereto withoutdeparting from the spirit and scope of the present invention as setforth in the appended claims.

1. A preparation method of a multi-component composite membranecomprising steps of: a) preparing a precursor film by injection of apolymer, which is used, for a support layer into an extruder; b)annealing the precursor film at a temperature less than a melting pointof the polymer; c) coating both surfaces of the precursor film with apolymer solution, which is used for an active layer; d) drying thecoated precursor film; e) low temperature-stretching the dried precursorfilm at a temperature less than room temperature; f) hightemperature-stretching the low temperature-stretched precursor film at atemperature less than the melting point of the polymer; and g)heat-setting the high temperature-stretched precursor film under tensionat a temperature less than the melting point of the polymer.
 2. Thepreparation method according to claim 1, wherein the polymer solution ofstep c) is coated on both sides of the precursor film by dip-coating. 3.The preparation method according to claim 1, wherein a concentration ofthe polymer solution of step c) is equal to or greater than 0.01 wt %.4. The preparation method according to claim 1, wherein the drying ofstep d) is performed at a relative humidity ranging from 1 to 100%. 5.The preparation method according to claim 1, wherein the drying of stepd) is performed under saturated vapor pressure.
 6. The preparationmethod according to claim 1, wherein the drying of step d) is performedunder a gas atmosphere selected from the group consisting of nitrogen,oxygen, carbon dioxide, and air atmosphere.
 7. The preparation methodaccording to claim 1, wherein an active layer having a thickness in therange of 0.1 to 20 μm is formed through the coating and drying of stepsc) and d).
 8. The preparation method according to claim 1, which furthercomprises the step of applying ion beams to either or both surfaces ofthe annealed precursor film with reactive gas between the steps b) andc).
 9. The preparation method according to claim 8, wherein the ion beamirradiation is performed by activation of electrons and a gas selectedfrom the group consisting of hydrogen, helium, oxygen, nitrogen, carbondioxide, air, fluorine, neon, argon, krypton, N₂O, and a mixture thereofsuch that the gas has an energy ranging from 0.01 to 10⁶ keV; andirradiating the surface of the precursor film with the ion beams. 10.The preparation method according to claim 8, wherein the ion beamirradiation amount ranges from 10⁵ to 10²⁰ ions/cm².
 11. The preparationmethod according to claim 8, wherein the ion beam irradiation isperformed under a gas atmosphere selected from the group consisting ofhelium, hydrogen, nitrogen, ammonia, carbon monoxide, carbon dioxide,chlorofluoro methane, methane, and N₂O atmospheres, and mixturesthereof.
 12. The preparation method according to claim 11, wherein theflow rate of the reactive gas ranges from 0.5 to 20 ml/minute.
 13. Thepreparation method according to claim 8, wherein the ion beamirradiation is performed under a vacuum ranging from 10⁻¹ to 10⁻⁶ torr.14. A preparation method of a multi-component composite membranecomprising steps of: a) annealing a precursor film comprising a polymerat a temperature less than a melting point of the polymer; b) coatingboth surfaces of the precursor film with a polymer solution, which isused for an active layer; c) drying the coated precursor film; d) lowtemperature-stretching the dried precursor film at a temperature lessthan room temperature; e) high temperature-stretching the lowtemperature-stretched precursor film at a temperature less than themelting point of the polymer; and f) heat-setting the hightemperature-stretched precursor film under tension at a temperature lessthan the melting point of the polymer.
 15. The preparation methodaccording to claim 14, wherein the polymer solution of step b) is coatedon both sides of the precursor film by dip-coating.
 16. The preparationmethod according to claim 14, which further comprises the step ofapplying ion beams to either or both surfaces of the annealed precursorfilm with reactive gas between the steps a) and b).
 17. The preparationmethod according to claim 16, wherein the ion beam irradiation isperformed by activation of electrons and a gas selected from the groupconsisting of hydrogen, helium, oxygen, nitrogen, carbon dioxide, air,fluorine, neon, argon, krypton, N₂O, and a mixture thereof such that thegas has an energy ranging from 0.01 to 10⁶ keV; and the surface of theprecursor film with the ion beams.